Inversor
Módulos FV y sistemas de almacenamiento de energía funcionan con corriente continua (CC), pero debido al ventajas de la corriente alterna (CA) la mayoría de los aparatos producidos en el mundo están construidos para funcionar con una fuente de entrada de CA. Esto significa que es común incorporar un inversor, que puede convertir de CC a CA, en cualquier sistema que esté destinado a funcionar con algo más que cargas básicos como iluminación, teléfonos celulares y radios. El término inversor cubre muchos productos diferentes con diferentes funcionalidades y costos, por lo que es importante comprender los diferentes factores importantes en la elección de un inversor para determinar el tipo ideal para cada aplicación.
Los inversores suelen ser el componente electrónicamente más complejo de un sistema FV autónomo, lo que significa que es probable que sean un punto de falla y que invertir en un inversor de calidad es una buena decisión. Si el inversor en un sistema basado completamente en CA falla, el sistema dejará de funcionar por completo. Por esta razón, muchos diseñadores de sistemas de pequeños sistemas autónomos eligen incorporar iluminación de CC o un refrigerador de CC en un sistema para ofrecer un sistema más robusto que continuará proporcionando estas funciones básicas incluso en el caso de una falla del inversor.
El inversor para un sistema fuera de la red debe dimensionarse y seleccionarse en función de evaluación de cargas para un sitio en particular; consulte Dimensionamiento y selección del inversor para más información.
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Inverter/cargador
Muchos inversores autónomos más grandes, llamados inversores/cargadores, pueden aceptar la entrada de CA de un generador y convertirla a CC para poder cargar el sistema de almacenamiento de energía si las condiciones climáticas son adversas. Esta puede ser una inversión inteligente para sistemas más grandes, ya que el tamaño de fuente FV y sistema de almacenamiento de energía no necesita ser tan grande para poder para satisfacer la demanda durante los períodos poco frecuentes de mal tiempo. Además, un inversor puede jugar un papel importante, ya que un sistema con un interruptor de bypass permitirá que el generador alimente directamente cargas de CA si el sistema FV no funciona correctamente o necesita desconectarse para mantenimiento. Se deberá elegir un inversor/cargador de acuerdo con el tamaño y tipo de generador que se utilizará.
Potencia de salida (Watt/VA)
Todos los inversores se clasifican según la potencia que pueden suministrar continuamente en watts o voltios/amperios. Además de esta clasificación de potencia continua, un inversor podrá suministrar mayores cantidades de energía durante breves períodos de tiempo para suministrar cargas que requieran picos momentáneos de más corriente al arrancar. Estas clasificaciones adicionales pueden venir en clasificaciones de 30 minutos, 5 minutos, 1 minuto, 30 segundos, 10 segundos o 1 segundo. Todas las cargas y sus requisitos de energía deberán evaluarse para seleccionar el inversor adecuado; consulte Dimensionamiento y selección del inversor para obtener más información.
DC input voltage
Inverters are typically available in 12 V, 24 V, or 48 V. Inverters with a smaller power rating will typically be available in 12/24 V configurations and larger inverters will be available in 24/48 V configurations.
AC output voltage
Inverters are manufactured for use in specific geographic markets given the voltage of the local grid, but is necessary to confirm that the specifications of the inverter - especially if importing the product - conform to the local voltage. Common output voltages for off-grid inverters are 120 V, 220 V, 240 V.
Frequency
The frequency of the grid varies globally between 50 Hz-60 Hz. An off-grid inverter that matches the local grid specifications should be chosen.
Waveforms
The most important characteristic of an inverter - that helps to define its functionality and quality - is the waveform of its alternating current output. The AC that the grid supplies comes in a pure sine wave, which is what all AC appliances are designed to use as their input. A smooth variation between directions of current flow that is operating is necessary for the proper functioning of various complex appliances, but for other simpler appliances it doesn't matter. The three types of output wave forms that are available in the market are the following:
- Pure sine wave (PSW): An inverter that outputs AC in a sine wave that is indistinguishable from that supplied by the electricity grid. The creation of a pure sine wave requires a more complex inverter design that costs more, but the additional cost of a pure sine wave inverter often bring additional efficiency and quality. It is recommended that any system that relies on AC continuously to supply loads incorporate a PSW inverter.
- Modified sine wave (MSW): An inverter that outputs AC in a waveform that is rougher than a pure sine wave, but that is indistinguishable for most appliances. MSW are a more economical option for small systems that require AC, but that are only going to supply simple loads (cell phones, radios, lights etc.) Should not be considered if a system is intended to be used with certain types of loads - motors, laser printers, battery chargers, washing machines, high-end music equipment - as it can cause them to work improperly or damage them. Motors will consume roughly 25% more energy with a MSW inverter compared to a PSW inverter and the life of the motor will be shortened as that extra energy will be converted into heat. If a system doesn't rely on AC continuously, but only periodically for smaller loads, then it is an option that should be considered.
- Square wave: The simplest and cheapest form of inverter. Current direction switches very rapidly and can damage certain appliances. Will work fine with simple loads like cell phones and lighting, but not recommended for use in a PV system. Frequently poorly designed and manufactured. A modified sine wave or pure sine wave inverter will not cost very much more.
Idle consumption
An inverter requires energy even if it is not currently supplying loads. Larger off-grid inverters may require more than 30W when not supplying loads, smaller inverters tend to require around 4-8 W. Some inverters will include a low consumption standby mode to reduce idle consumption, but standby modes often do not function well with small off-grid systems as the inverter only activates when a load of a sufficient size is connected to the system. A cell phone charger will likely not wake the inverter from standby mode.
Inverter idle consumption can greatly affect the design of smaller PV systems as a constantly operating inverter may be the most energy intensive load that the system supplies. It is common practice with smaller systems that use DC for lighting and cell phone charging to incorporate an inverter that is only used as needed to reduce the size of the PV source.
| Daily idle consumption | = idle watts × hours of operation per day |
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Example 1: A small off-grid PV system incorporates an 800W inverter that consumes 7W of power as it sits idle. How much energy will it consume if left on continuously?
- Daily idle consumption = 7 W × 24 hr
- Daily idle consumption = 168 W
This is more energy than two efficient 3W LED lightbulbs - a common size in off-grid applications - would consume if left on continuously.
Efficiency
Inverters vary in terms of how efficiently the transform DC into AC. Many inverter manufacturers offer a maximum or peak efficiency number for their products, but this number is not likely to be achieved in practice. Inverters will only achieve these efficiency numbers when under a sufficient load, but the number drops rapidly with less loading. An off-grid inverter in practice will likely be somewhere in the 85-90% efficiency range. The efficiency of an inverter can have a significant impact on system design and performance.
| Total energy demand | = energy demand of loads ÷ inverter efficiency |
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Example 1: Two inverters are being considered for an off-grid PV system. Both inverters have an 800W power rating, but one is 90% efficient and the other is 85% efficient. It is anticipated that the inverter will have to supply 2400Wh of energy each day. How much extra energy will each inverter require to meet this demand?
- Total energy demand inverter 1 = 2400 Wh ÷ .85 = 2824 Wh
- Total energy demand inverter 2 = 2400 Wh ÷ .90 = 2667 Wh
The less efficient inverter will require 157 Wh more of energy each day to supply the loads. This could mean that the PV source needs to be larger.
Additional inverter features
There are many other additional features that inverters offer that may be of value on a specific project.
User interface
The user interface is important as it can conveys vital information about the loads and the state of the inverter, which users need to revise regularly in order to be able to adjust their usage properly and protect the battery bank. Additionally, a user interface should be assessed for how much programming it allows the user to perform and if it allows the revision of historical system data.
Programmability
The larger the power rating of an off-grid inverter, typically the more user programming is permitted to enable customization according to the end user needs. There are basic functions, like the set point for the low voltage disconnect, and other more complicated functions related to its output, standby modes to save on idle consumption, generator input and monitoring. See inverter programming for more information.
Data logging/monitoring
A data logging/monitoring system can enable an inverter to share or record data about the performance of the system. The level of detail and amount of time for which an inverter can store data varies. Information about maximum power, usage and system voltage can be very useful in assessing how the system is performing in that location, if the user is treating the system properly and resolving any technical issues that may arise. Some systems may also offer the capability of remote monitoring through cell phone signals or the internet, which can be very useful in remote off-grid applications if possible.
Projected life
There is no specific projected life for an inverter as it varies significantly based upon its quality and how it is used. A low-quality inverter may only last for six months of heavy use before failing, whereas a high-quality charge controller used lightly could last decades. With inverters you generally get what you pay for.
Maintenance
The user manual for an inverter should always be consulted, but most inverters do not require much maintenance if they are used under proper conditions. They should be kept free of dust, insects, and water. Connections should be periodically revised - at least once a year - to make sure that they are still tightened properly and not creating unnecessary resistance.
Recyclability
Inverters contain a variety of different materials and chemicals that can be hazardous if not disposed of properly. They should be treated as electronic waste. Contacting the manufacturer is recommended.
